1. Field of the Invention
The invention relates to a surface treatment bath which applies chemical dissolution and/or deoxidation sulfurization for articles formed of a heat-resistant nickel alloy. It is particularly adapted for such alloys containing between 3.5 and 10% of molybdenum.
2. Brief Description of the Prior Art
During the fabrication of parts for turbojetengines and the like, for instance prior to welding operations, it is necessary to remove the metal oxide surface layer formed on heat-resistant alloy parts. The surface layer of such oxides is usually formed on the articles during the course of preceding thermal treatment of the articles.
It is also necessary to remove the surface layer of turbojetengine parts which have been in operation for hundreds of hours before such parts may be repaired. During their normal course of operation, the parts are exposed to corrosive, high-temperature gases which form oxysulfides containing the oxides of the metals from which the parts are made and sulfides of various origins.
The conventional procedures for eliminating such hot-formed oxides involve alkaline degreasing followed by decarbonization in an acid or alkaline medium. A subsequent oxide treatment may involve a medium of potassium permanganate plus potassium hydroxide, or may be accomplished by passing the article through a molten soda bath. The final phase of the treatment process involves removing the residual oxides and scouring by immersion in a nitrohydrofluoric acid.
However, it has been found that these baths are ineffective when parts made of NC22KDA, NC25D, or Z10CNKDW20 alloys were pickled and the nitrohydrofluoric acid bath produced undesirable intergranular corrosion. An analysis of these typical alloys indicated that they contained a somewhat high proportion of molybdenum; NC22KDA, commercially called INCO617 contains 8-10% molybdenum; NC25D, commercially known as NIMONIC86 contains 10.65% molybdenum; Z10CNKDW20, commercially known as HA556 includes 2.5-4% molybdenum; NK17CDAT, commercially known as ASTROLOY contains 4.5-5.5% molybdenum; and NC14K8 commercially known as RENE95, contains 3.5% molybdenum.